performed under ambient conditions. Other closely related methods have also
been introduced, including desorption atmospheric pressure chemical ioniza-
tion (DAPCI) (Takats et al., 2005a) and electrospray-assisted laser desorption
ionization (ELDI) (Shiea et al., 2005).
11.2.2.5 Direct Analysis in Real-Time (DART) In DART (Fig. 11.2b), a
plasma of excited-state atoms/molecules, ions, and electrons are generated by
an electrical discharge of several kilovolts applied to a gas (helium or nitrogen).
The gas flows into a second chamber where ions and electrons are removed by
a perforated electrode. The gas flow then passes through a third region that can
be optically heated. The exiting gas is directed toward a liquid or solid sample
surface. The electronic or vibronic excited-state species (metastable helium
atoms or nitrogen molecules) are the working reagent that desorb and ionize
low molecular weight molecules from the surface (Cody et al., 2005). Different
ionization mechanisms occur depending upon the nature of the carrier gas,
analyte, and polarity of ions (Cody et al., 2005). Although not yet extensively
used for metabolite profiling and identification, this technique could be useful
in identifying low molecular weight, non-polar metabolites.
11.2.3 Mass Analyzers
The function of a mass analyzer is to measure them/zratios of ions. There are
five principal types of mass analyzers in common use today that can be divided
into two groups: beam-type mass analyzers and ion-trapping mass analyzers
(McLuckey and Wells, 2001). In beam-type analyzers, the ions leave the ion
source in a beam and pass through the analyzing (electric or magnetic) field to
the detector. Beam-type mass analyzers include time-of-flight, sector magnets,
and quadrupole mass filters. In trapping-type analyzers, ions are trapped in the
analyzing field after being formed in the analyzer itself or being ejected from
external ion source (Glish and Vachet, 2003). Trapping analyzers includes ion
traps, Fourier transform ion cyclotron resonance (FTICR) and FT-Orbitrap
mass spectrometers.
11.2.3.1 Time-of-Flight (TOF) Analysis in a TOF mass spectrometer is
based on the principle that ions of differentm/zvalues, when accelerated by the
same kinetic energy, possess different velocities after acceleration out of the ion
source and into a field-free drift tube (Guilhaus et al., 2003; Weickhardt et al.,
1996). As a result, the time (t) required for each ion to traverse the flight tube is
different and high mass ions will take longer to reach the detector than low
mass ions. The equation relating the flight time of an ion with itsm/zvalue is
shown below:
t¼
L
v
¼
L
ffiffiffiffiffiffi
2 zV
m
q ¼L
ffiffiffiffiffiffiffiffi
m
2 zV
r
LC/MS INSTRUMENTATION 325